1. Field of the Invention
The present invention relates to a catalyst, an electrode, a membrane electrode assembly, and fuel cell.
2. Description of the Related Art
In a fuel cell, a fuel such as hydrogen and methanol is electrochemically oxidized within the cell so as to convert the chemical energy of the fuel directly into an electric energy so as to take out the electric energy. In thermal power generation, NOx or SOx is generated by the combustion of the fuel. However, the fuel cell is free from the generation of SOx or NOx and, thus, attracts attention as a supply source of a clean electric energy.
In the fuel cell, a catalyst prepared by allowing active metal particles such as particles of platinum, ruthenium or a platinum ally to be carried by a carrier such as a carbon carrier is used in general as the catalyst of the cathode and the anode. Platinum is used as the active catalyst component because platinum exhibits a high oxidizing activity of the fuel and a high reducing activity of oxygen. Also, carbon is used as the carrier of the active catalyst component because carbon is electrically conductive and, thus, is considered to be effective for taking out electrons generated on the surface of the active catalytic metal particle.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2001-283865 discloses a catalyst prepared by allowing a noble metal catalyst such as platinum, palladium, gold, ruthenium, or iridium to be carried by a conductive particle such as carbon black.
The electrode catalyst prepared by allowing platinum to be carried by carbon black exhibits excellent characteristics as described above. However, where the electrode catalyst is prepared by allowing metals other than platinum, e.g., by allowing gold or a gold alloy, to be carried by carbon black, a difficulty is brought about that the power generation efficiency of the fuel cell is lowered. Under the circumstances, it is of high importance to develop a catalyst carrier that can be used in place of carbon black, i.e., a catalyst carrier other than carbon black, which permits carrying gold and a gold alloy in addition to platinum so as to permit the fuel cell to exhibit a high power generation efficiency.
It is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-8797 that fine gold particles are fixed to metal oxides containing titanium as the main component such as TiO2, FeTiO3, CaTiO3 and SrTiO3 in order to obtain simply and with a high efficiency an electrode catalyst excellent in the catalytic activity.
However, since the metal oxides disclosed in the publication quoted above are insulating materials, it was impossible for the fuel cell to exhibit an excellent power generation efficiency.
On the other hand, Jpn. Pat. Appln. KOKAI Publication No. 2003-151567 teaches that a perovskite type oxide is used in the air electrode of a fuel cell as a catalyst for catalyzing the decomposing reaction of hydrogen peroxide so as to produce oxygen. The perovskite type oxide noted above includes, for example, Cux-1SrxTiO3 (x=0 to 0.5), LaxSr1-xMnO3 (x=0 to 0.5), and SrTiO3.
Further, Jpn. Pat. Appln. KOKAI Publication No. 2003-308846 discloses an air electrode for a fuel cell. It is taught that the air electrode is formed of a perovskite type oxide. The perovskite type oxide has a general formula of “A(A1A2)BO3”, in which a rare earth element is contained in the A1 site, an alkaline earth metal is contained in the A2 site, and cobalt is contained in the B site.